When readers search for misspellings in a proofreading task or for a letter in a letter detection task, they are more likely to omit function words than content words. However, with misspelled words, previous findings for the letter detection task were mixed. In two experiments, the authors tested the functional equivalence of both tasks. Results reveal similar patterns of omissions for both search tasks when misspellings were produced by substituting one letter for another: There were more omissions for function words than for content words. However, when two letters were transposed, differences between the two tasks emerged. It is possible to conclude that letter detection and proofreading are based on common cognitive processes but the nature of the target can modulate the effects. Results are interpreted in light of the attentional disengagement model and the guidance organization model of the missing-letter effect.